In an optical communication system, it is required to stabilize an operation of a light source, such as a semiconductor laser and so on. For example, in an optical integrated circuit used as an optical transmitting/receiving module, in the case that an optical coupler is coupled to a light source, reflected light is generated at the optical coupler. If the reflected light is inputted to the light source, the operation of the light source will become unstable.
Thus, in a prior-art technique, an optical isolator is placed outside an optical integrated circuit chip. Patent-related Document 1 describes that, when placing a PLC (Planar Lightwave Circuit) and a bulk optical component, or placing two PLCs, with a certain distance therebetween when constructing a spatial optical system in an optical signal processing device, it is necessary to insert an optical isolator between the PLCs for preventing change of device characteristics due to reflected light returning to an output-end surface of the PLC.
Also, in a prior-art technique, an optical input/output end surface of an optical integrated circuit chip is set to have a slope with respect to a surface vertical to the direction of propagation of light. Patent-related Document 2 describes an optical element chip in which a PLC waveguide and an LN (lithium niobate) waveguide are optically coupled, wherein the connecting surface of the PLC waveguide and the LN waveguide is set to have a slope with respect to an optical axis.
Patent-related Document 3 describes a technique to place an optical isolator in an optical transmission module, and a technique to set a plane of incidence from a laser light source to an optical member to be slanted with respect to an optical axis.
However, the above constructions result in increase of costs and increase of optical loss.